In magnetic recording media, for example as used in hard disk drives, information is written to and read from magnetic elements that represent digital bits. The minimum size that can be written to is called a cluster, and cluster size defines media recording density. Clusters may vary in size. Bits may include a plurality of magnetic clusters, e.g. seven to fifty clusters may equal one bit.
Just as magnetic clusters are the smallest building blocks for magnetic patterns written to a medium, the physical grains are the smallest physical building blocks. In present media, grains are polycrystalline units consisting of a single chemical composition (e.g. CoCr), separated by a boundary consisting of a different composition. Clusters may contain one physical grain or a number of grains. Grains behave like conversations at a party. A few loner grains may remain by themselves, or more interesting grains may group together in larger numbers. Therefore a single grain may be a cluster, or a number of grains may group together and form a cluster. Individual grains may be visually analyzed using microscopic instruments. However, the microscopic instruments that are capable of sensing magnetic fields (for example, transition electron microscopy) have great difficulty observing magnetic clusters in recording media. Other methods such as magnetic force microscopy suffer from poor spatial resolution, and cannot directly observe magnetic clusters.